Progressing cavity pumps typically have a single threaded screw, termed a rotor, located inside a stator having a double threaded cavity located therein. The rotor and the stator are shaped to create cavities along the length of the pump. As the rotor is rotated within the stator, the cavities progress from an inlet end of the pump to an outlet or discharge end. Thus, rotation of the rotor inside the stator pumps material located in the pump from the inlet end to the outlet end.
Stators may be formed of or coated with an elastomeric material to ensure a strong seal between the stator and rotor. The rotor and elastomeric stator form a compressive fit therebetween which allows the progressing cavity pump to self-prime, suction lift fluids (i.e. pump against gravity) and pump against a pressure (i.e., pump against a back pressure). However, stators lined with elastomeric material may have a performance disadvantage, especially when pumping moderate-to-high viscosity fluids due to pressure limitations of the elastomeric materials and frictional forces between the rotor and the stator.
Stators that are not lined with an elastomeric material (also known as “rigid stators”) may be formed of relatively rigid materials such as steel. Progressing cavity pumps having rigid stators may have a gap or clearance between the rotor and the stator. The clearances between the rotor and stator reduce friction and allow for more efficient pumping of moderate viscosity fluids (i.e. having a viscosity of between about 3000 centipoise and about 20,000 centipoise) and high viscosity fluids (i.e. having a viscosity of greater than about 20,000 centipoise). In particular, when pumping moderate-to-high viscosity fluids, the viscous fluids fill the gaps or clearances between the rotor and stator to allow efficient pumping operations. However, the gap between the rotor and the stator may prevent the pump from being self-priming, can limit its ability to suction lift fluids, and may limit its volumetric efficiency, especially when pumping relatively low viscosity fluids (i.e. having a viscosity of less than about 300 centipoise, or less than about 100 centipoise, or between about 0.5 centipoise and about 100 centipoise).
Accordingly, there is a need for a progressing cavity pump, and in particular, a stator for use with a progressing cavity pump which can be self-priming, and can create sufficient suction and can pump against high pressure, while providing efficient pumping operations.